Luminescence studies of ZnO crystals and nanowires
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ZnO is a direct semiconductor with a band gap of 3.4 eV at room temperature making it a hot topic for optoelectronic research across a broad range of applications. The current solid state lighting technology typically uses nitride semiconductors in the generation of light, more commonly gallium nitride. ZnO is a more efficient light generator than GaN owing to its high excitonic binding energy, and for this reason, ZnO is a potential material that may soon compete with GaN as a cornerstone of the solid state lighting revolution. Significant obstacles preventing the wide scale usage of ZnO include the lack of reliable p-type doping and high degree of uncertainty surrounding the nature of its defects, intrinsic n-type conductivity and optical properties. The aim of this thesis is therefore to explore the luminescence and defect properties of doped and undoped ZnO nanowires and crystals. During the proj ect, ZnO nanowires were grown through a vapour deposition method under varying growth conditions. Changes in the choice of substrate, gas flows , pressures, and growth times were linked to changes in the structural and optical properties of the nanowires as characterised by scanning electron imaging and complementary spectroscopic techniques. Gold coated epitaxially matched sapphire substrates positioned close to the source material were found to produce highly aligned nanowires arrays. Cathodoluminescence (CL) imaging showed a localisation of defect luminescence near the surface of ZnO nanowire sidewalls. Oxygen deficiencies were also found to be localised on the sidewalls of the nanowires, supporting a correlation between green luminescence and oxygen vacancies in ZnO. Post processing plasma modification of ZnO crystals and powders were used to ident ify defects contribut ing to the observable green luminescence. The defect emissions were fitted with constrained Gaussian peaks which were linked to multiple competitive radiative centres. Variations in the near band edge (NBE) to green defect intensity ratios were also investigated to assist in the assignment of the defect peaks. Incorporation of transition metals into ZnO was achieved through thermal in-diffusion and sol-gel preparation methods. Significant quenching of the defect related optical emissions relative to the UV emission was observed for both Mn doped samples, while an enhancement of the defect emission was observed near the surface of Fe doped crystals. Monochromatic CL imaging was shown to be an effective method of determining the depth of iron incorporation in iron doped ZnO crystals owing to the enhancement of the green emission.
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